Numerous agents have been described which will inhibit the complement system. These include heparin, suramin, epsilon-aminocaproic acid, and tranexamic acid. However, no orally effective agents have been described that will leave the necessary opsonization of the classical complement pathway functional, but which will prevent self-damage either by blocking C3 convertase activity of the alternative pathway, as well as assembly of the membrane attack complex by both pathways. The only approved agent for treating aberrant complement activation is eculizumab, a humanized monoclonal antibody which blocks C5 conversion of the alternative pathway. It has been approved for the treatment of paroxysmal nocturnal hemoglobinemia. It is effective in 49% of cases (Hillmen et al. 2006). However it does not block the earlier step of C3 convertase, which can result in ongoing hemolysis of erythrocytes (Parker 2012). Moreover, as a high MW immunoglobulin antibody, it will not cross the blood brain barrier and will not be effective in CNS disorders.
The present inventors show in this invention that components of less than 1 kDa MW of the aurin tricarboxylic acid synthetic complex (ATAC) block C3 convertase of the alternative pathway, as well as membrane attack complex (MAC) assembly at the final stage of C9 addition to C5b8 of both the alternative and classical pathways. The present inventors further show that they are safe and effective following oral administration.
Complement is a key component of both the innate and adaptive immune systems. It carries out four major functions: recognition of a target for disposal, opsonization to assist phagocytosis, generation of anaphylatoxins, and direct killing of cells by insertion of the membrane attack complex (MAC) into viable cell surfaces. Although complement is an essential defense system of living organisms, it is widely regarded as a two edged sword. Its opsonizing components are beneficial, but the membrane attack complex is potentially self-damaging.
The complement system as it is understood today is illustrated in FIG. 1. It consists of two main pathways: the classical and the alternative. The pathways have differing opsonizing mechanisms, but they have in common assembly of the terminal components to form the membrane attack complex (C5b-9). The classical pathway commences with the C1q component of the C1 complex recognizing a target that needs to be phagocytosed. Subsequent steps involve dissociation of the C1 complex, cleavage of C2, C4, and C3 to provide amplification as well as covalent attachment of the activated complement components to the target. By this means the target is disposed of by phagocytes that have receptors for the activated complement components so attached.
Both pathways result in C5 being cleaved into C5a and C5b. The released C5b fragment can then insert itself into the membranes of nearby cells. C6, C7, C8 and C9 (n) can then become sequentially attached to the membranes. The addition of C9 renders the complex functional by opening holes in the membranes, thus leading to death of the cells. Its physiological purpose is to kill foreign pathogens, but in the case of sterile lesions, it can destroy host cells by the phenomenon known as bystander lysis.
The complement system therefore operates in two parts. The first part is opsonization, which prepares targeted tissue for phagocytosis. The second part is assembly of the membrane attack complex, which has the purpose of killing cells. The former is essential, but the latter is not. For example, approximately 0.12% of Japanese are homozygous for the nonsense CGA-TGA (arginine 95stop) mutation in exon 4 of C9 (Kira et al., 1999). These individuals cannot make a functioning membrane attack complex. This means that there are more than 150,000 Japanese leading healthy lives despite this deficiency. The Japanese experience indicates that selective inhibition of membrane attack complex formation on a long term basis is a viable therapeutic strategy.
The membrane attack complex exacerbates the pathology in all diseases where there is persistent overactivity of the complement system. In addition, pathology can be exacerbated in diseases in which there is alternative pathway C3 convertase over activity. Such diseases include, but are not limited to, rheumatoid arthritis, paroxysmal nocturnal hemoglobinemia, multiple sclerosis, malaria infection, Alzheimer disease, age-related macular degeneration, and atherosclerosis. The purpose of this invention is to provide a method for successfully treating such conditions. The present inventors screened a large library of organic compounds for any that might have promise of being a selective inhibitor of these pathways. Commercially supplied ‘aurin tricarboxylic acid’ was the only material to pass the initial screening test. The present inventors found that the product contained only a small amount of aurin tricarboxylic acid. It consisted mostly of a complex of high molecular weight materials. The present inventors fractionated the crude material and investigated the properties of components of less than 1 kDa MW. The desired properties were identified in true aurin tricarboxylic acid (ATA, MW422), aurin quadracarboxylic acid (AQA, MW572), aurin hexacarboxylic acid (AHA, MW858), and their combination which The present inventors term the low molecular weight aurin tricarboxylic acid complex (ATAC).